1. Field of the Invention
The present invention relates to a web printing paper coated on both sides, consisting of a coating base paper that contains mineral filler and, as fiber components, wood pulp and/or cellulose. The coating, which contains pigment and binder and serves to absorb the printing ink, is applied by means of a film press. The invention also relates to a process for manufacturing the web printing paper.
2. Description of the Prior Art
Web printing papers of the aforementioned type are used in the production of periodicals, especially illustrated periodicals, magazines, catalogs and advertising brochures, such as those normally inserted into daily newspapers or illustrated periodicals.
Numerous attempts have already been made to reduce production costs for the papers in question, particularly by reducing the grammage of the coating base paper or by replacing the cellulose and/or wood pulp fibers with recycled fibers.
An overview of the offset papers containing wood that are commonly used in Europe is found in the article "Study of Binder Systems for LLWC and MFP Papers" ("Studie ueber Bindemittelsysteme fuer LLWC and MFP Papers") published in the "Paper Manufacturing Weekly" ("Wochenblatt fuer Papierfabrikation") 9 (1988), pp. 337 to 344. This article also describes a film press used to apply preparations to the paper types in question. The described pigment composition consists of a mixture of 60 parts of English China Clay and 40 parts of a fine American kaolin. The authors of this article conclude that when an SD coater (short dwell time coating device) is used, 20 parts by weight of binder are required for an application of 5 g/m.sup.2, while 30 parts by weight are needed for an application of 3 g/m.sup.2. However, if a film press is used instead of an SD coater, it is considered necessary to increase the binder level in order to achieve the same printing gloss, albeit with lower opacity.
In the "Paper Manufacturing Weekly" 16 (1990), pp. 701 to 708, and "Pulp and Paper Canada" 92:4 (1991) pp. 52-58, there are reports on the surface treatment of newsprint paper by means of a film press. In these studies, a base paper was used that had an ash content of only 0.4% by weight. No information is given about the fineness of the pigments used for the surface treatment. In the formulas used for the surface pigmentation, the extraordinarily high binder content of 70% by weight, relative to the pigment, is notable. The studies are essentially limited to a coat application weight of 5 g/m.sup.2 per page or side.
EP-0 377 983 A2 relates to a newsprint paper coated by means of an air brush or a roller coating device, which is said to have improved ink absorption, surface strength and opacity. Coat weights between 1 and 12 g/m.sup.2, preferably between 3 and 8 g/m.sup.2, per side to be printed are disclosed, whereby a total grammage of 60 g/m.sup.2 is not exceeded. The filter content of the known coating base paper lies in the usual range of between 0.5 and 10% by weight, preferably, however, at less than 0.5% by weight.
In the "Paper Manufacturing Weekly" 1 (1988) pp. 1 to 6, the article "Surface Coated SC Papers, a Challenge for LWC Papers," relates to coating SC papers with a filler content of 15 to 25%. As the application device, an SDTA application system (short dwell time applicator) is reported. The application weights are in the range up to 3.5 g/m.sup.2 per side, while in the case of coating base papers with a lower filler content of less than 10% by weight, higher application weights are required--for example, in the so-called LWC papers, a coating application weight of up to approx. 9 g/m.sup.2 per side.
According to the article: "Possibilities for the On-Line Coating of Filled, Wood-Containing Papers" (Moglichkeiten zum On-Line Streichen von gefullten, holzhaltigen Papieren) in the "Paper Manufacturing Weekly" 13 (1992), pp. 507-515, highly-filled upgraded SC papers are initially pretreated by means of a film press with a pigmentation consisting of calcium carbonate and at least 30% by weight binder, relative to the pigment, so that the papers have adequate strength to be coated with the actual top coat in a second workstep by means of a blade-coater. The top coats consist of mixtures of kaolin and fine calcium carbonate, whereby 12 parts by weight binder to 100 parts by weight pigment mixture are used. The minimum application quantity for the cover coat is 7 g/m.sup.2 per side; by way of example, reference is made to application weights up to a total of 19 g/m.sup.2, while for the pre-coat 4 to 5 g/m.sup.2 are indicated.
Although the produced papers, which represent a new type of coated paper quality, have excellent characteristics in part, disadvantages exist insofar as the required double coat and the high weight of the top coat result in a paper which, due to its production costs, must remain excluded from significant areas of use. The article also describes in detail the disadvantages of film splitting at the output of a film press.
The article "Surface Treatment of Printing Papers containing AP--a Challenge on the Way to New Paper Qualities," ("Oberflaechenbehandlung von AP-haltigen Druckpapieren--eine Herausforderung auf dem Weg zu neuen Papierqualitaeten") published in the "Paper Manufacturing Weekly" 2 (1993), pp. 53 to 61, reports on various coating devices, including the film press known as the "speedsizer." Furthermore, in respect to the surface treatment of SC paper, a pigment layer of 4 to 5 g/m.sup.2 is described as adequate to attain good gravure printing quality; however, it is noted in limitation of this that in the case of higher application weights to improve coverage and whiteness, a marked film splitting results when a speedsizer is used and leads to a drastic reduction in gloss. For this reason, a second coating step with a blade-coating device is considered useful in order to attain good gravure printing properties, although this admittedly results in increased machine expenditure and additional operating costs and thus makes the product more expensive.
Finally, the article "Experiences with Twin HSM in the Surface Finishing of Base Papers based on Waste Paper," ("Betriebserfahrungen mit Twin HSM bei der Oberflaechenveredelung von Rohpapieren auf Altpapierbasis"), published in the "Paper Fabrication Weekly" 21 (1993), pp. 886 to 890, reports on the problems associated with the use of a film press. This article also mentions high application weights; however, in respect to the type of applied coaters it discloses only that these were pigmented compositions.